The present invention relates generally to implantable pulse generators for treating cardiac arrhythmias, and more specifically to a safety timeout for use with a method for discrimination between supraventricular and ventricular tachyarrhythmias.
One of the most frequent causes of follow-ups for patients with an implantable cardioverter-defibrillator (ICD) is inappropriate therapy. Between 15% and 30% of ICD patients receive inappropriate therapies because of supra-ventricular tachycardias (SVT). Single chamber defibrillators have algorithms such as morphology discrimination, sudden onset and interval stability that can be used to differentiate between ventricular tachycardias (VT) versus those that originate supra-ventricularly, i.e., in the atria. These algorithms have provided increased specificity for SVTs while still maintaining a high level of sensitivity to VT, but inappropriate therapies can still occur.
With the advent of dual-chamber defibrillators that sense both in the atrium and ventricle, atrial rate has been used to aid in the identification of VTs and SVTs. This has further increased the specificity for SVTs, but relying solely on the relationships between the rates of atrial and ventricular sensed events can result in erroneous classifications. Existing dual-chamber ICDs have exhibited difficulty in diagnosing certain rhythms correctly such as sinus tachycardia (tach), SVTs with 1:1 ventricular conduction, and VT with 1:1 retrograde conduction. VT with 1:1 retrograde conduction may look like sinus tach if the retrograde conduction interval is quite long. Conversely, sinus tach with first degree AV block can be classified by some schemes as a retrograde VT. Ventricular tachycardia during an atrial tachycardia has also been erroneously detected as SVT. Various diagnostic qualifiers can be implemented for improved discrimination of SVT from VT where the atrial and ventricular rates are the same or when the atrial rate is greater than the ventricular rate. However, they present the possible problem of extending the time required before a diagnosis is made and therapy is delivered. This is particularly problematic where a ventricular tachycardia is hemodynamically compromising or may degrade to ventricular fibrillation. It is desirable to provide a safety backup in the discrimination algorithm to ensure therapy is delivered where appropriate.
The present invention provides a timer that is used to trigger the delivery of therapy when no diagnosis is made within a defined time during a tachycardia episode. In a diagnostic algorithm where one or more rhythm discriminators or qualifiers such as morphology, sudden onset, interval stability and AV association is enabled, the timer starts as soon as a ventricular interval or interval average is faster than a programmed maximum time to diagnosis (MTD) cutoff. This trigger may be further limited to an SVT overlap zone beyond which the rapid ventricular rate is automatically considered a VT. If the timer times out before the algorithm diagnoses a particular rhythm, a prescribed therapy is delivered. In the preferred embodiment, the therapy that is triggered is a function of the most recent detected ventricular rate and the programmed MTD therapy.
In a preferred embodiment of the invention, a rate of ventricular contractions is sensed from a patient""s heart. If the ventricular rate exceeds a tachycardia threshold but is less than an SVT overlap zone upper limit, an episode timer is started. The sensed heart rhythm is analyzed using at least one arrhythmia qualifier to determine if a ventricular tachycardia should be diagnosed. If the presence or absence of a ventricular tachycardia is not diagnosed by the time the episode timer reaches an MTD cutoff, a programmed therapy is delivered.